Chair base



FIG. 1 is a front, left perspective view of a chair base showing my new design;

FIG. 2 is a bottom, left perspective view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a left side view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof. 

The ornamental design for a chair base, as show and described. 